Relax ActiveModel dependency

I tested it with 3.0.0, which is the earliest release.

I had to change "TestClass" to "TestModel" because it thought the class
name was plural.

    1) ActiveInteraction::ModelFilter.prepare(key, value, options = {}, &block) with options[:class] as a valid String with the right class returns the instance
       Failure/Error: expect(result).to eql value
       NameError:
         uninitialized constant TestClas
       # ./lib/active_interaction/filters/model_filter.rb:33:in `constantize'
       # ./lib/active_interaction/filters/model_filter.rb:21:in `prepare'
       # ./spec/support/filters.rb:6:in `block (2 levels) in <top (required)>'
       # ./spec/active_interaction/filters/model_filter_spec.rb:18:in `block (5 levels) in <top (required)>'